Marking instrument



FIG. 1 is a front elevational view of a marking instrument showing my new design;

FIG. 2 is a left side elevational view, the right side being a mirror image thereof;

FIG. 3 is a rear elevational view thereof;

FIG. 4 is a top plan view thereof; and

FIG. 5 is a bottom plan view thereof. 

The ornamental design for a marking instrument, as shown and described. 